How to make a facebook App?

[M Anthony]

I've had an idea for a website. I won't forget about it, but nonoe seems to care. But that's what's supposed to happen.

Instead of making a site.. I suppose making it do it's function through facebook would work great too.
I just read Facebooks builder's room mate who decided to not get in on facebook back in 2004, (his 400 million dollar mistake) now has a app that makes allot of money and supports charities

How are they made? and connected to facebook?.
It says you can make one on there, but can it be Complex??